Router bits for control cavity

Discussion in 'Luthier's Corner' started by JLW, Nov 21, 2016.

  1. JLW

    JLW

    Dec 5, 2006
    San Francisco, CA
    I used the search function, but did not find anything that fit my criteria.


    I bought the top bearing router bits you see on stewmac (I believe it's 1/2" x 1/2"). I think the collet size is 1/4" for the router I own. However, they only go to a depth of about 1 1/4", maybe a little less.

    I need a bit that can go to a depth of around 1 3/4" or even 2". Top-bearing. Does such a thing exist with only a 1/4" collet size?

    I have examples of this for bits that have a 1/2" collet size, but none that have 1/4" collet size.

    Obviously I would be doing the routing in several shallow passes, the reason why I need such a great depth is because I need it for a control cavity that would go through the entire body (and then put a top over the body)
     
  2. HaMMerHeD

    HaMMerHeD

    May 20, 2005
    Last edited: Nov 21, 2016
  3. rudy4444

    rudy4444

    Mar 13, 2012
    Central Illinois
    They (and lots of other places) sell the 1/2" O.D. bearing separately. I added the bearing to a 1-1/4" cutting length bit and that takes care of all my pattern routing needs. I make extra-thick templates and then remove the template and route to the just formed cavity if I need additional depth. If you make your patterns thick there's no reaason to limit your cutting length to 1/2", although you have to go slower because the additional length can cause chatter on end grain cuts.
     
    Radio likes this.
  4. INTP

    INTP

    Nov 28, 2003
    Dallas, TX
    I'm thinking that a 2" cutting depth on a 1/4" shaft bit is asking for trouble. Best case, the bit would flex and give an uneven cut. Worst case is that it would snap at 20,000 RPM and become a sharp high-velocity missile.
     
  5. rudy4444

    rudy4444

    Mar 13, 2012
    Central Illinois
    The cutting depth is really 1-1/2" maximum, and that is achieved by taking successive 1/8" or 1/4" passes. I've been doing it that way for several years without any problem using a Porter Cable 690, YMMV.
     
  6. JLW

    JLW

    Dec 5, 2006
    San Francisco, CA
    Even if you were doing very shallow 1/4" deep passes at a time? Obviously I wouldn't cut all 2" at once.
     
  7. Hopkins

    Hopkins Supporting Member Commercial User

    Nov 17, 2010
    Houston Tx
    Owner/Builder @Hopkins Guitars
    Means2nEnd likes this.
  8. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    JLW;

    Hopkins is right; that's the bit you want, if you only have a router with a 1/4" collet. They don't make long router bits with 1/4" shanks for a reason; they would be unsafe to use.

    A quick general note about routing: You mentioned a very shallow 1/4" deep cut per pass. That's too deep for most routers! With a 1/2" diameter bit, take no more than 1/8" per pass. Stick with that rule for safe, happy routing. I'm not kidding. Trying to cut too deep is the biggest cause of scary router stories. If you have a big heavy 3 1/4 HP router with a 1/2" shank bit and a rigid fixture setup, then you can think about 1/4" per pass.

    If you aren't clear on the procedure for routing a deep cavity using that bit, first make your template from 3/4" thick plywood or MDF. That's important, it needs to be 3/4" thick. Stick or clamp it down onto the body. Set the router down onto the template (shut off) and zero it. That is, bring the bit down until it's just contacting the body surface, and zero the adjustment dial. Lift the router up, adjust it down 1/8", start it, tip it down in, and go around the template, routing the whole cavity 1/8" deep. Repeat, cutting 3 or 4 more passes of 1/8" deep each until you run out of vertical travel of the router. Watch that the collet doesn't hit the template.

    Then, remove the template. Set the router right on the body surface. Re-zero it. Now, with the first 1/8" deep cut, the bearing should be riding on the side wall of the cavity. Continue on, 1/8" deep passes until you reach the final depth.

    By the way, if you plan to cut the cavity all the way through the body wood, using a separate top, it's usually neater to glue the top on first, and then rout the cavity from the back.
     
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  9. JLW

    JLW

    Dec 5, 2006
    San Francisco, CA
    Great, thanks Bruce. Your insight is much appreciated and I will definitely do shallower passes from now on. On my first build I routed the neck pocket as well as the cavity in increments that were just under 1/4", but I'll stick to 1/8" just to be on the safe side.
     
  10. T_Bone_TL

    T_Bone_TL

    Jan 10, 2013
    SW VT
    For a through the body cavity you could also route just enough to match your cover, drill a hole, thread a coping saw blade through, and cut the bulk out with the coping saw. For any cavity, you can remove bulk wood fast with Forstner bits using a drill.
     
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  11. rudy4444

    rudy4444

    Mar 13, 2012
    Central Illinois
    Most definitely. I always hog out control cavities, pickup routes, etc. with Forstners.
     
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  12. JLW

    JLW

    Dec 5, 2006
    San Francisco, CA
    I'm assuming you mean using an electric coping/scroll saw? Or do you mean a hand coping saw? Would a hand coping saw be any faster than just routing?
     
  13. T_Bone_TL

    T_Bone_TL

    Jan 10, 2013
    SW VT
    I mean the hand-powered kind. The powered ones are not particularly faster than using a hand one correctly. Either kind cuts badly and/or breaks with a dull blade in it, or if you push it too hard rather than letting the blade cut. You are turning FAR less wood into sawdust making a sawcut around the perimeter than routing your way down from the top - and if you are staying within reasonable limits for a 1/4" shaft router, the sawing might well be faster.
     
  14. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    I agree about hand coping saws. Once you get a little practice with one, it can make many kinds of complicated cuts with surprisingly little effort. Like T_Bone said, it's removing a small amount of wood, so it cuts quickly.

    Coping saws are also great for cutting aluminum, using the standard wood cutting blades. It'll easily cut openings in 1/4" aluminum plate.

    Yes, a Forstner bit is about the fastest way to remove a bunch of wood from a cavity. If you are working with a small (1 1/2 hp or less) size router, hogging out the bulk of the wood with a Forstner bit before routing will save you time. With a big router rig, like a 3 1/4 hp hand router or a 7 1/2 hp pin router, then the routing alone will be a little faster.
     
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  15. Randyt

    Randyt RAAPT Custom Wood Productions

    Jul 21, 2010
    Barrie, Canada
    i have found using a Drill press and a Forstner bit the best for removing material..the drill press will get you very close to finish depth...and consistent across bottom of cavity...the center of bit drops an 1/8" so leave it up to be taken out with router bit later...I do all four corners with 1/4" forstner bit first though...this way I can use a 3/8" or larger router bit (top bearing/bottom flush) to route out all the rest of material.(theres NOT much there)..Ive never had a router bit slip/ bite/ chip etc. on me!!!
     
  16. T_Bone_TL

    T_Bone_TL

    Jan 10, 2013
    SW VT
    Knock wood after you say that...
     
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  17. Randyt

    Randyt RAAPT Custom Wood Productions

    Jul 21, 2010
    Barrie, Canada
    Haha...yes...good idea!!!